Since about the year 2000, Controlled Source ElectroMagnetism (“CSEM”) has grown from the research stage to commercial applications for oil and gas marine exploration (Constable & Srnka, Geophysics 72, WA3-WA12 (March-April 2007)). Typically, in offshore CSEM, a vessel tows a submerged CSEM source (also called transmitter) above the sea floor. Electric and/or magnetic fields are recorded at receivers lying on the sea floor. Conventionally, negative offsets correspond to the distances between the transmitter locations and the receiver when the transmitter is approaching the receiver. Positive offsets correspond to the distances between the receiver and the transmitter locations when the transmitter is moving away from the receiver. Processing transforms the recorded time series of the electric and magnetic fields into frequency domain data at discrete frequencies. The high frequencies in the source signal are attenuated in their journey from source to receiver more quickly than the low frequencies. Thus, the high frequencies do not penetrate as deep as the low frequencies, but, in the other hand, the resolution of the low frequency data is poorer. The decay of the electric or magnetic fields with offset is controlled by the resistivity of the subsurface. A conductive earth induces a more rapid decay. Resistivity anomalies like oil or gas accumulations produce anomalously slower decays. CSEM interpretation consists in detecting and making sense of those anomalies.
With increasing experience around the world, it has appeared that the earth resistivity structure is more complicated than initially anticipated. More data are required to reduce the interpretation ambiguity. Nowadays, it is common to use denser surveys (more lines and more receivers) and several frequencies to improve lateral and vertical resolution. The technical problem addressed by the present invention is how to overcome the daunting problem of displaying a huge amount of data in a meaningful way.
It is difficult to detect the presence of an anomaly from a first glance at conventional parametric plots such as FIGS. 1A-B, which show the survey data recorded at one receiver from one line of towed transmitter. The X-axis (i.e. the horizontal axis) corresponds to the distance between the receiver and the transmitter locations (offsets). The location of the receiver corresponds to the X=0 line. The Y-axis (i.e. the vertical axis) of FIG. 1A corresponds to the amplitude of the electric field (note the logarithmic scale) and the Y-axis of FIG. 1B corresponds to the phase of the electric fields. Magnetic field data would yield similar pictures. The parameter in FIGS. 1A-B is frequency. (Several frequencies can be extracted from typical survey data.) Data for six frequencies ranging from 0.125 Hz (101) to 2 Hz (102) are shown (arrow 103 indicating the direction of increasing frequency), but many more frequencies can be extracted from waveforms used in modern surveys. In the case of FIGS. 1A-B, there is a known, small anomaly that extends to the right (positive offsets) of the receiver, 600 meters below the sea floor. At a first glance it is difficult to tell that the right sides of the plots look more resistive, or any different at all, than the left sides. Plotting several frequencies only increases the confusion on the plot.
Since the data interpreter is looking for anomalies, it is convenient to estimate or simulate the normal, i.e. background, electro-magnetic response and to compare the recorded data to this reference. FIGS. 2A-C show the variation of the electric field at four receivers (the tip locations of the inverted “V's” correspond to receiver locations, also indicated by reference number 205) at 3 different frequencies (0.125 Hz for FIG. 2A, 0.25 Hz for FIG. 2B and 2 Hz for FIG. 2C) along one tow line. The darker points correspond to the amplitude of the recorded fields and the lighter-shade lines to the simulation of a reference earth, without anomaly (background simulation). From the high frequency data (2 Hz, FIG. 2C), it is obvious that the measured data are above the simulated curves on the right side of the plot (201). This indicates a resistivity anomaly. A close examination of the lower frequency plots (FIGS. 2A-B) also show a more resistive character on the right sides of the plots (202) and (203), but the response is much less obvious (lack of resolution). This way of displaying data is obviously not very satisfactory and can be very confusing if many receivers and frequencies are considered.
The picture can be simplified by considering only the ratio between the observed data and the reference (S. Ellingsrud, et al., The Leading Edge 21, 972-982, (2002)). FIGS. 3A-C show the ratio of the actual data amplitude to the reference amplitude for the same four receivers (1, 2, 3 and 4) at three frequencies (the same frequencies as in FIGS. 2A-C) along a tow line. The receiver reference number is also used to indicate the amplitude ratio curve corresponding to that receiver. Left of receiver 4 (region 301), the ratio is very close to 1, i.e. there is no anomaly. Right of receiver 4 the maximum ratio varies from greater than 2 at 2 Hz (best resolution, region 302) to smaller than 1.25 at 0.125 Hz (lowest resolution, region 303). It may be noted that the high frequency data do not extend as far as the low frequency data because they are more quickly attenuated and rapidly fall below the noise level. The edge of the resistivity anomaly is located right below receiver 4 (the anomaly extends to the right), but the ratio is not significantly greater than one at the location of receiver 4. Accurately picking the edge of the anomaly is difficult. Three different plots are required to show the information at the three different frequencies. Plotting all data together on one figure would make a very confusing picture.
Other publications dealing with improved ways of displaying or interpreting CSEM data include US Patent Publication US/2006/0197534 and PCT International Publication WO 2006/096328.